The original sender, in a frame travelling between two access points.
The only address field that is sometimes not there, and the reason the header has no fixed length. When a frame crosses between two pieces of infrastructure, the transmitter and the sender are different devices and so are the receiver and the destination — so all four roles need their own field and this one appears. Every other combination of the DS bits gets by with three, and the six octets are simply absent: not zero, not reserved, not there, with QoS Control moving up to meet Sequence Control.
Address 4
Authored in QoS Data frame . Drawn in 11 frames.